docs: def california_basin_model(vs30):
      """
      Returns the centred z1.0 (mu_z1) based on the California model
      (equation 11)
      """
      coeff = 570.94 ** 4.0
      model = (-7.15 / 4.0) * np.log(
          ((vs30 ** 4.0) + coeff) / ((1360.0 ** 4.0) + coeff)
          ) - np.log(1000.)
      return np.exp(model)